JPH05124894A - Production of carbon crystal thin film - Google Patents

Abstract

PURPOSE:To obtain the (100) face oriented crystal thin film of a carbon crystal (fluorene crystal) by maintaining a single crystal substrate of RbBr, KI or RbI at a prescribed temp. and heating a carbon vapor deposition source (fluorene C60) to execute heteroepitaxial growth under a vacuum. CONSTITUTION:Raw materials of the (fluorene C60) are packed in a tantalum heater and the (100) cleavage plane substrate of the RbBr is set at about 5cm distance from a vapor deposition source; thereafter, the inside of a vacuum chamber is evacuated down to 1X10<-6>Torr. The vapor deposition for about one hour is executed by maintaining the substrate temp. at 10 to 60 deg.C and the raw material heating temp. at 300 to 400 deg.C. As a result, the bluish yellow thin film is obtd. on the (100) cleavage plane substrate. The film thickness is about 3000Angstrom . The result obtd. by investigating the crystallinity by an X-ray diffraction method reveals that the thin film is the (100) face oriented crystal thin film of the fluorene crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high quality crystalline thin film of carbon allotrope molecule fullerene (C ₆₀ ).

[0002]

2. Description of the Related Art Diamond (three-dimensional lattice), graphite (two-dimensional lattice) and carbine (one-dimensional lattice) are known as substances composed only of carbon element. Recently, a new spherical cluster molecule consisting of 60 carbon atoms has been found, and its physical properties and applications are of great interest. This molecule has a soccer ball-like structure consisting of 12 pentagonal faces and 20 hexagonal faces, and is called fullerene (C ₆₀ ). (H.W.Kr
oto et al .: Nature 318 (1985) 16
2. )

X-ray diffraction experiments have revealed that fullerene molecules (C ₆₀ ) form molecular crystals (fullerite) having a face-centered cubic lattice (space group: Fm3, lattice constant = 14.17 Å) as an aggregate. Furthermore, it has been confirmed that superconducting characteristics are exhibited by doping this crystal with an alkali metal element (K, Rb). Its phase transition temperature is
It is reported that K doping is about 18 K and Rb is about 28 K, which is the highest value excluding oxide high temperature superconducting materials. (A.F. Hebard et al .: Nature 350
(1991) 600. M .; J. Rosseinsky
Et al: Phys. rev. Lett. 66 (1991)
2830. However, the samples used for these physical property measurements are polycrystals or polycrystal thin films consisting of crystal grains with a grain size of several tens of microns, and in order to investigate more detailed physical properties, single crystal or high quality crystal thin films Is required.

As a method for producing fullerene (C ₆₀ ), graphite is used in an atmosphere of rare gas such as argon or helium (10
0 to 500 Torr), it is evaporated by YAG laser irradiation or arc discharge, the generated soot is dispersed as a raw material in an organic solvent such as benzene or toluene, filtered, and the filtrate is dried to form a polycrystalline powder. obtain. However, at this stage, because they also include C ₇₀ composed of 70 carbon atoms with C _60, separated by chromatography or the like, it is necessary to purify. On the other hand, in order to obtain a fullerene (C ₆₀ ) thin film, a method of sublimating raw material soot or purified C ₆₀ in a vacuum or a rare gas and depositing it on a substrate is used. Since the fullerene (C ₆₀ ) molecule has a spherical shape, and all its bonds are exhausted by intramolecular bonds, the interaction between molecules is only van der Waals force. Therefore, when deposited on an amorphous substrate such as glass, it is expected to have a fine structure and have a hexagonal array on the substrate surface. In fact, a hexagonal lattice arrangement of C ₆₀ molecules has been confirmed by STM (scanning tunneling microscope) on a gold substrate. (R. J. Wils
on et al .: Nature 348 (1990) 621. )
However, confirmation of this alignment film is also a result on a microscopic scale, and there is no example of producing an alignment crystal film over a wider area. Therefore, in order to control the orientation of the fullerene crystal and obtain a large-area crystal film, it is important to search for a substrate material that can realize heteroepitaxial growth.

Heteroepitaxial growth of molecular crystals is more difficult than that of inorganic crystalline materials because the interaction between molecules and the substrate is generally small. However, with some molecular organic crystals such as phthalocyanine, heteroepitaxial growth on an ionic crystal substrate such as potassium bromide (KBr) has been realized. This is because the atomic arrangement pattern on the substrate surface and the period match well with those of the phthalocyanine crystal. That is, for the success or failure of heteroepitaxial growth, it is important to thoroughly examine the matching of the crystal structures of the substrate material and the material to be formed thereon.

[0006]

Polycrystalline materials are used for measuring the physical properties of flan crystals. Therefore, in the measurement of the phase transition temperature in the superconducting characteristic experiment, the high value of 28 K is obtained from the Meissner effect, whereas it is difficult to determine the clear phase transition temperature from the electrical resistance measurement. It is considered that this is because the high quality crystal has not been obtained and therefore the doping is not performed uniformly.
Therefore, clarifying the detailed physical properties of fullerenes,
In order to provide electrical and optical applications, it is desired to establish a technique for producing a high quality oriented crystal film.

[0007]

According to the present invention, a single crystal substrate of rubidium bromide (RbBr), potassium iodide (KI) or rubidium iodide (RbI) is used in an amount of about 10 to 60.
Carbon deposition source (raw material: fullerene C ₆₀ )
Of (100) -oriented crystal thin film of fullerene crystal (C ₆₀ ) on the (100) face of the substrate by heating the substrate to about 300 to 400 ° C. and performing heteroepitaxial growth under vacuum. Regarding the method. As a result of earnest studies, in the present invention, by selecting an appropriate substrate material, it was found that a high quality and controllable orientation can be achieved and a high quality crystal thin film can be produced. First, the crystal structure of a fullerene crystal was examined, and a heteroepitaxial substrate material was selected.

As such a substrate material, an alkali halide which is an ionic crystal having a rock-salt type structure was examined.
As a result of examining the lattice matching based on the lattice constant, among many alkali halides, rubidium bromide (RbBr), potassium iodide (KI) and rubidium iodide (RbI) were suitable. The (100) plane of these crystals has a lattice mismatch with the (100) plane of the fullerene thin film of 2.8% for rubidium bromide (RbBr), 0.3% for potassium iodide (KI), and iodide. Rubidium (RbI) is -3.6%, which is a small value that can be expected to realize sufficient teloepitaxial growth. FIG. 1 is a crystal structure of a fullerene crystal at room temperature. (PA Heiney: Phys.
Rev. Lett. 66 (1991) 2911. ) C ₆₀ molecules are shown here as spheres. The size of the C ₆₀ molecule itself is about 7Å in diameter, and the crystal structure has a lattice constant a = 1.
4.17 Å face centered cubic lattice (space group Fm bar 3). On the other hand, FIG. 2 shows a crystal structure of K ₃ C ₆₀ obtained by doping a fullerene crystal with potassium. This substance is T _C
It exhibits superconducting properties below 18K. From the figure, the basic lattice constant created by C ₆₀ molecules by doping is a = 14.24.
It has only slightly increased to Å. Space group is F
It is m bar 3. The doped potassium atom is C ₆₀
It is in the gap of the lattice made by and the structure of the whole crystal is that the square lattice of potassium (length = a / 2) is inserted in the z = 1/4 and 3/4 planes of the rock salt structure. It is thought that it has become. Considering the above results, a / 2 is used as the substrate.
If a cubic crystal having a period of the period is used, a fullerene crystal and an alkali metal element-doped K ₃ C ₆₀
It can be presumed that it is possible to grow a heteroepitaxial thin film crystal such as Rb ₃ C ₆₀ or Rb ₃ C ₆₀ . By using (100) plane such as RbBr, KI and RbI as a substrate material, it has become possible to obtain a high quality fullerene crystal (100) plane oriented thin film which has never been obtained.

As described above, the fullerene raw material as the vapor deposition source is a soot generated by evaporating graphite in a rare gas atmosphere (100 to 500 Torr) such as argon or helium by YAG laser irradiation or arc discharge. Use C ₆₀ powder purified from. C ₆₀ for example made of Texas Fullerene Corp. commercially can be suitably used. The substrate temperature during this manufacturing is preferably about 10-60 ° C. If the temperature is 10 ° C. or lower, the agglomerates are nonuniformly aggregated on the substrate surface, and if the temperature is 60 ° C. or higher, polycrystallization occurs, which is inconvenient. Further, the temperature of the vapor deposition source of carbon fullerene is preferably about 300 to 400 ° C. Three
If it is below 00 ° C, the vapor deposition rate will be low, and if it is above 400 ° C, the crystallinity will be poor.

[0010]

EXAMPLES Examples of the present invention will be described below.

Example 1 A commercial product (C ₆₀ manufactured by Texas Fullerene Co., Ltd.) was used as a fullerene (C ₆₀ ) raw material, and a (100) cleaved surface of a rubidium bromide (RbBr) single crystal was used as a substrate material. A vacuum deposition method was used as the film method. The vacuum vapor deposition apparatus incorporates a tantalum boat heater as a vapor deposition method, a tungsten heater for heating as a substrate holder, and a copper water cooling tube for cooling. The temperature of the vapor deposition source and the temperature of the substrate are each monitored by a thermocouple. Also,
The vacuum chamber is evacuated by a turbo molecular pump.

About 10 mg of fullerene (C ₆₀ ) raw material was charged into a tantalum boat heater, and rubidium bromide (RbBr) was added.
After setting the (100) cleaved surface substrate of No. 3 at a distance of about 5 cm from the vapor deposition source, the inside of the vacuum chamber was evacuated to 1 × 10 ⁻⁶ Torr.
The substrate temperature is 10 ° C. to 60 ° C. (50 ° C. in this embodiment).
The appropriate heating temperature is 300 ° C to 400 ° C (350 ° C in this embodiment). As a result of vapor deposition at a substrate temperature of 50 ° C. and a raw material temperature of 350 ° C. for about 1 hour, a bluish, yellow thin film was obtained. The film thickness is about 3000
It was Å. The crystallinity of the obtained thin film was examined by an X-ray diffraction method.

FIG. 3 shows an X-ray diffraction pattern. Substrate RbBr
In addition to (200) reflection of fullerene crystals,
And (600) reflections are observed. This result shows that the obtained fullerene thin film is (100) oriented crystal as expected and heteroepitaxial growth is realized. Rubidium iodide (Rb
Similar results were obtained when I) and potassium iodide (KI) were used. It is needless to say that the heating method of the substrate and the raw material is not limited to this embodiment, and it is also possible to use a Knudsen cell or the like.

[0014]

According to the present invention, fullerene crystal (C ₆₀ )
Since a high-quality oriented crystal thin film can be obtained, accurate physical properties such as electrical conductivity and optical properties due to doping and the like can be measured, and progress in application of fullerenes is promoted.

[Brief description of drawings]

FIG. 1 Room temperature crystal structure of fullerene crystal (C ₆₀ ).

FIG. 2 Room temperature crystal structure of superconductor (K ₃ C ₆₀ ).

FIG. 3 is an X-ray diffraction pattern (X-ray source: CuKα, raw material temperature 350 ° C., substrate temperature 50 ° C., film thickness 3000 Å) of the thin film crystal produced in the example.

Claims (1)

[Claims] 1. A ruthenium bromide (RbBr), potassium iodide (KI) or rubidium iodide (RbI) single crystal substrate is held at about 10 to 60 ° C., and a carbon vapor deposition source (raw material: fullerene C ₆₀ ) is used. Is heated to about 300 to 400 ° C., and heteroepitaxial growth is performed under vacuum to form (10) of fullerene crystal (C ₆₀ ) on the (100) plane of the substrate.
0) A method for producing a carbon crystal thin film, which comprises forming a plane-oriented crystal thin film.